SPACE CENTER, Houston — In a numbing setback sure to set off a national debate over the future of the space program, NASA has grounded all future shuttle flights because of a large chunk of foam that broke off Discovery’s fuel tank in hauntingly similar fashion to Columbia’s doomed mission.

This time, engineers believe, the foam tumbled harmlessly away during liftoff and Discovery was spared.

“Until we’re ready, we won’t go fly again,” shuttle program manager Bill Parsons said Wednesday evening at a grim news conference. “I don’t know when that might be, so I’ll just state that right up front. We’re just in the beginning of this process of understanding.”

NASA Administrator Michael Griffin promised the space agency would make any needed modifications before shuttles lift off again. He stressed that this is a test flight and “among the things we are testing are the integrity of the foam insulation and the performance of new camera equipment installed to detect problems.”

“The cameras worked well. The foam did not,” he said in a statement.

NASA does not believe the flying debris that peeled off the external fuel tank struck Discovery. Every indication so far, officials said, is that the shuttle is safe to return its seven astronauts to Earth.

“Call it luck or whatever, it didn’t harm the orbiter,” Parsons said. If the foam had broken away earlier in flight — when the atmosphere is thicker, increasing the acceleration and likelihood of impact — it could have caused catastrophic damage to Discovery.

The loss of such a large chunk of debris, a vexing problem NASA thought had been fixed, shattered the euphoria from Tuesday’s shuttle launch, the first in 2½ years. The redesign of the fuel tank was the focal point of the space agency’s $1 billion-plus effort to make the 20-year-old space shuttles safer to fly following the 2003 Columbia tragedy.

The grounding also adds to the burden on the international space station, which has been relying solely on Russia’s much smaller spacecraft for crew and cargo deliveries.

“It doesn’t appear that the mission is in jeopardy. Nothing is in jeopardy except the schedule. But I don’t want to underestimate the seriousness of it in terms of the future,” Boehlert said.

The three remaining shuttles are due to retire in 2010, and a new spacecraft is in the works. President Bush has a lofty plan for NASA to return astronauts to the moon by 2020 and eventually to Mars. It’s unclear how the latest grounding might affect public sentiment for the space program.

The piece of foam flew off Discovery’s redesigned tank just two minutes after what initially looked like a perfect liftoff, right after the booster rockets peeled away. But in less than an hour NASA had spotted images of a mysterious object whirling away from the tank.

Mission managers did not realize what the object was — or how much havoc it would cause — until Wednesday after reviewing video and images taken by just a few of the 100-plus cameras in place to watch for such dangers.

Parsons offered no excuses, saying, “You have to admit when you’re wrong. We were wrong.”

Discovery’s astronauts were informed of the foam loss Wednesday, but made no mention of it over the NASA radio lines after they awakened late at night and began preparing for Thursday’s linkup with the space station.

The station’s two residents had cameras ready to zoom in on Discovery from 600 feet out, to check for any damage from the foam or other debris. The plan called for the shuttle to perform an unprecedented back flip to expose its entire belly to the station photographers.

Engineers believe the irregularly sized piece of foam that came off was 24 to 33 inches long, 10 to 14 inches wide, and between 2 and 8 inches thick — only somewhat smaller than the 1.67-pound chunk that smashed into Columbia’s left wing during liftoff. The plate-sized hole let in superheated gases that caused the shuttle to break up on its return to Earth on Feb. 1, 2003.

On Discovery, the foam broke away from a different part of the tank than the piece that mortally wounded Columbia.

Atlantis — whose own fuel tank is now suspect — was supposed to lift off in September, but that mission is now on indefinite hold. Parsons refused to speculate when a shuttle might fly again, but did not rule out the possibility that Discovery’s current mission may be the only one for 2005.

Parsons said it was unlikely Atlantis would be needed for a rescue mission, in the event Discovery could not return safely to Earth and its astronauts had to move into the space station. Discovery, fortunately, appears to be in good shape for re-entry, he said.

Wednesday’s inspection of Discovery’s wings and nose using a new 100-foot, laser-tipped crane turned up nothing alarming, but analysis was ongoing, Hale said.

In addition to the big chunk of foam, several smaller pieces broke off, including at least one from an area of the fuel tank that had been modified after Columbia. Thermal tile was also damaged on Discovery’s belly soon after liftoff; one tile lost a 1½-inch piece right next to the set of doors for the nose landing gear, a particularly vulnerable spot.

Hale said none of the tile damage looked serious and likely would not require repairs in orbit.

Imagery experts and engineers expect to know by Thursday afternoon whether the gouge left by the missing piece of tile — or anything else — needs another look. The astronauts’ inspection boom could determine precisely how deep the damage is, and they will probably pull it back out Friday.

The 12 orbiting astronauts on the International Space Station were hailed by the chief on Thursday — and also got a humorous scolding for keeping their android crewmate locked up in its packing for so long.